Lec 55: Pharmacokinetics I Flashcards Preview

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Flashcards in Lec 55: Pharmacokinetics I Deck (12):

Zero order kinetics

constant rate of absorption or elimination


-speed is constant, rate of absorptionis independent of concentration gradient
-transport mechanism is saturated


First order kinetics

-rate is proportional to the amount of drug still unabsorbed

-EXPONENTIAL, M= Mo(e)^(-kt)

-increase difference:increase speed

-[ ] is major determinant, gradient counts!
-not constant amount, constant fraction


order Applications: Drug Absorption/Drug Elimination

zero order elimination - duration of a therapeutically effective drug [] increases linearly with amount in body fluids
first order elimination - duration of a therapeutically effective drug [] increases as the log of the amount in the body fluids does


half life equation

half life = 0.693/ke
ke=rate constant of elimination


Volume of Distribution

-apparent Volume in which the drug is distributed (a concept, not a real number)

Vd=(mg drug in body)/([]in plasma mg/L)

-as plasma [] decreases, Vd increases
-can be much higher than the total body volume if drug is nonhomogenously distributed with high [] in extravasculartissues (plasma [ ] is low in this case)



-expressed in L/hr or mL/min

-Volume of plasma cleared of drug

-CL is additive

CL=(rate of elimination in mg/hr)/([] drug in mg/L)

half life=0.693*Vd/CL



-fraction of unchanged drug reaching systemic circulation after oral administation (F)


ER= extraction ration in liver
f=fraction absorbed from gut


Extent of absorption

ratio of drug entering blood circulation to total drug in digestive system
rate= extent *time


First Pass Elimination

presystemic hepatic elimination; passes through liver before system circulation and may be metabolized or excreted into bile

-drug is decreased by : QCi - QCo, where Q= 90 L/h hepatic blood flow and Ci=drug entering liver, and Co = drug leaving

Hepatic CL= Q*ER

***if ER is large, we can compensate for low oral bioavailability by 1) increasing dose and 2) use alternative administative routes


accumulation factor of a drug when the dosing interval is shorter than four half-live

accumulation factor = 1/(fraction lost in one dosing interval)

example: for a drug given once every half life, AF=2


Dosing Regimen Construction

1.identify target [] that will produce desired effect - [] range between that causing ½ of greatest therapeutic effect (lower limit) and [] that will cause side effects in no more than 5-10% of all patients (uppe rlimit)

2.determine Vd and CL

3.maintenance dose

a.dosing rate = CL*Css , where Css= []at steady state

4.if intermittent doses are given, then maintenance dose is determined by:maintenance dose=dosing rate* dosing interval

5.if target [] needs to be reached quick, and or elimination rate ke is small, loading doses might be applied: loading dose= Vd*Target []

6.check for desired effect and adjust


Relate half lives to steady state

It takes 4-5 half lives to reach/change a steady state

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